The function of 33-kDa protein in the photosynthetic oxygen-evolution system studied by reconstitution experiments

Treatment of Photosystem II particles with 1.2 M CaCl₂ released three proteins of 33, 24 and 18 kDa of the photosynthetic oxygen evolution system, but left Mn bound to the particles as demonstrated by Ono and Inoue (Ono, T. and Inoue, Y. (1983) FEBS Lett. 164, 252-260). Oxygen-evolution activity of the CaCl₂-treated particles was very low in a medium containing 10 mM NaCl as a salt, but could be restored by the 33-kDa protein. When the particles were incubated in 10 mM NaCl at 0°C, two of the four Mn atoms per oxygen-evolution system were released with concomitant loss of oxygen-evolution activity. The 33-kDa protein suppressed the release of Mn and the inactivation during the incubation. These findings from reconstitution experiments suggest that the 33-kDa protein acts to preserve Mn atoms in the oxygen-evolution system. The 33-kDa protein could be partially substituted by 100 or 150 mM Cl^- for the preservation of the Mn and oxygen-evolution activity. The Mn in Photosystem II particles enhanced rebinding of the 33-kDa protein to the particles.